1. Field of the Invention
The invention relates generally to the field of marine geophysical surveying. More particularly, the invention relates to methods for controlling the spatial distribution or geophysical data gathering equipment towed behind a survey vessel.
2. Background Art
Marine geophysical surveying systems such as seismic acquisition systems and electromagnetic survey systems are used to acquire geophysical data from formations disposed below the bottom of a body of water, such as a lake or the ocean. Marine seismic surveying systems, for example, typically include a seismic survey vessel having onboard navigation, seismic energy source control, and geophysical data recording equipment. The seismic survey vessel is typically configured to tow one, or more typically a plurality of laterally spaced apart sensor streamers through the water. At selected times, the seismic energy source control equipment causes one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals generated by various sensors on the one or more streamers in response to detected seismic energy are ultimately conducted to the recording equipment. A record with respect to time is made in the recording system of the signals generated by each sensor (or groups of such sensors). The recorded signals are later interpreted to infer the structure and composition of the formations below the bottom of the body of water. Corresponding components for inducing electromagnetic fields and detecting electromagnetic phenomena originating in the subsurface in response to such imparted fields may be used in marine electromagnetic geophysical survey systems.
The one or more sensor streamers are in the most general sense long cables that have geophysical sensors disposed at spaced apart positions along the length of the cables. A typical streamer can extend behind the geophysical survey vessel for several kilometers.
Multiple streamer systems are used in what are known as three dimensional and four dimensional geophysical surveys. A four dimensional seismic survey is a three dimensional survey over a same area of the Earth's subsurface repeated at selected times.
The quality of geophysical images of the Earth's subsurface produced from three dimensional or four dimensional surveys is affected by how well the positions of the individual sensors on the streamers are controlled. Various devices are known in the art for positioning streamers laterally and/or at a selected depth below the water surface. U.S. Pat. No. 5,443,027 issued to Owsley et al., for example, describes a lateral force device for displacing a towed underwater acoustic cable that provides displacement in the horizontal and vertical directions.
U.S. Pat. No. 6,011,752 issued to Ambs et al. describes a seismic streamer position control module.
U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes a method for controlling the navigation of a towed seismic streamer using “birds” affixable to the exterior of the streamer.